Because of their widespread occurrence in water and plant seeds such asdicots, thepseudomonads were observed early in the history ofmicrobiology. The generic namePseudomonas created for these organisms was defined in rather vague terms byWalter Migula in 1894 and 1900 as a genus of Gram-negative, rod-shaped, and polar-flagellated bacteria with some sporulating species.[11][12] The latter statement was later proved incorrect and was due to refractive granules of reserve materials.[13] Despite the vague description, the type species,Pseudomonas pyocyanea (basionym ofPseudomonas aeruginosa), proved the best descriptor.[13]
Like most bacterial genera, the pseudomonad[note 1]last common ancestor lived hundreds of millions of years ago. They were initially classified at the end of the 19th century when first identified byWalter Migula. The etymology of the name was not specified at the time and first appeared in the seventh edition ofBergey's Manual of Systematic Bacteriology (the main authority in bacterial nomenclature) asGreekpseudes(ψευδής) "false" and-monas (μονάς/μονάδος) "a single unit", which can mean false unit; however, Migula possibly intended it as falseMonas, a nanoflagellated protist[13] (subsequently, the term "monad" was used in the early history of microbiology to denote unicellular organisms). Soon, other species matching Migula's somewhat vague original description were isolated from many natural niches and, at the time, many were assigned to thegenus. However, many strains have since been reclassified, based on more recent methodology and use of approaches involving studies of conservative macromolecules.[14]
16S rRNA sequence analysis has redefined the taxonomy of many bacterial species.[15] As a result, the genusPseudomonas includes strains formerly classified in the generaChryseomonas andFlavimonas.[16] Other strains previously classified in the genusPseudomonas are now classified in the generaBurkholderia andRalstonia.[17][18]
In 2020, a phylogenomic analysis of 494 completePseudomonas genomes identified two well-defined species (P. aeruginosa andP. chlororaphis) and four wider phylogenetic groups (P. fluorescens, P. stutzeri, P. syringae, P. putida) with a sufficient number of available proteomes.[19] The four wider evolutionary groups include more than one species, based on species definition by the Average Nucleotide Identity levels.[20] In addition, the phylogenomic analysis identified several strains that were mis-annotated to the wrong species or evolutionary group.[19] This mis-annotation problem has been reported by other analyses as well.[21] In 2021, a broad phylogenomic analysis on this genus led to the reorganization of the species included inPseudomonas, leading to the creation of several new genera to accommodate some of them.[22]
In 2000, the completegenome sequence of aPseudomonas species was determined; more recently, the sequence of other strains has been determined, includingP. aeruginosa strains PAO1 (2000),P. putida KT2440 (2002),P. protegens Pf-5 (2005),P. syringae pathovar tomato DC3000 (2003),P. syringae pathovar syringae B728a (2005),P. syringae pathovar phaseolica 1448A (2005),P. fluorescens Pf0-1, andP. entomophila L48.[14]
By 2016, more than 400 strains ofPseudomonas had been sequenced.[23] Sequencing the genomes of hundreds of strains revealed highly divergent species within the genus. In fact, many genomes ofPseudomonas share only 50–60% of their genes, e.g.P. aeruginosa andP. putida share only 2971 proteins out of 5350 (or ~55%).[23]
By 2020, more than 500 completePseudomonas genomes were available inNCBI GenBank. A phylogenomic analysis utilized 494 complete proteomes and identified 297 core orthologues, shared by all strains.[19] This set of core orthologues at the genus level was enriched for proteins involved in metabolism, translation, and transcription and was utilized for generating a phylogenomic tree of the entire genus, to delineate the relationships among thePseudomonas major evolutionary groups.[19] In addition, group-specific core proteins were identified for most evolutionary groups, meaning that they were present in all members of the specific group, but absent in other pseudomonads. For example, severalP. aeruginosa-specific core proteins were identified that are known to play an important role in this species' pathogenicity, such asCntL, CntM, PlcB, Acp1, MucE, SrfA, Tse1, Tsi2, Tse3, andEsrC.[19]
In 2021, a comparative genomic study with more than 3000Pseudomonas genomes helped to discover genes and functions related with the environmental adaptation of these bacteria.[8]
Pseudomonas may be the most common nucleator of ice crystals in clouds, thereby being of utmost importance to the formation of snow and rain around the world.[28]
The genusPseudomonas is recognized for its remarkable metabolic diversity, enabling it to thrive in a wide range of environments. These bacteria produce a vast array ofsecondary metabolites,[29] including antibiotics, siderophores, and biosurfactants, which contribute to their ecological versatility and biotechnological potential.
Allspecies and strains ofPseudomonas have historically been classified asstrict aerobes. Exceptions to this classification have recently been discovered inPseudomonasbiofilms.[30] A significant number of cells can produce exopolysaccharides associated with biofilm formation. Secretion ofexopolysaccharides such as alginate makes it difficult for pseudomonads to bephagocytosed by mammalianwhite blood cells.[31] Exopolysaccharide production also contributes to surface-colonisingbiofilms that are difficult to remove from food preparation surfaces. Growth of pseudomonads on spoiling foods can generate a "fruity" odor.[citation needed]
This ability to thrive in harsh conditions is a result of their hardycell walls that contain proteins known asporins. Their resistance to most antibiotics is attributed toefflux pumps, which pump out some antibiotics before they are able to act.[citation needed]
Pseudomonas aeruginosa is increasingly recognized as an emergingopportunistic pathogen of clinical relevance. One of its most worrying characteristics is its low antibiotic susceptibility.[32] This low susceptibility is attributable to a concerted action of multidrug efflux pumps with chromosomally encodedantibiotic resistance genes (e.g.,mexAB-oprM,mexXY, etc.[33]) and the low permeability of the bacterial cellular envelopes. Besides intrinsic resistance,P. aeruginosa easily develops acquired resistance either bymutation in chromosomally encoded genes or by thehorizontal gene transfer of antibiotic resistance determinants. Development ofmultidrug resistance byP. aeruginosa isolates requires several different genetic events that include acquisition of different mutations and/or horizontal transfer of antibiotic resistance genes. Hypermutation favours the selection of mutation-driven antibiotic resistance inP. aeruginosa strains producing chronic infections, whereas the clustering of several different antibiotic resistance genes inintegrons favours the concerted acquisition of antibiotic resistance determinants. Some recent studies have shown phenotypic resistance associated tobiofilm formation or to the emergence of small-colony-variants, which may be important in the response ofP. aeruginosa populations toantibiotic treatment.[14]
Althoughgallium has no natural function in biology, gallium ions interact with cellular processes in a manner similar to iron(III). When gallium ions are mistakenly taken up in place of iron(III) by bacteria such asPseudomonas, the ions interfere with respiration, and the bacteria die. This happens because iron is redox-active, allowing the transfer of electrons during respiration, while gallium is redox-inactive.[34][35]
Infectious species includeP. aeruginosa,P. oryzihabitans, andP. plecoglossicida.P. aeruginosa flourishes in hospital environments, and is a particular problem in this environment, since it is the second-most common infection in hospitalized patients (nosocomial infections).[36] This pathogenesis may in part be due to the proteins secreted byP. aeruginosa. The bacterium possesses a wide range ofsecretion systems, which export numerous proteins relevant to the pathogenesis of clinical strains.[37] Intriguingly, several genes involved in the pathogenesis ofP. aeruginosa, such asCntL, CntM, PlcB, Acp1, MucE, SrfA, Tse1, Tsi2, Tse3, andEsrC are core group-specific,[19] meaning that they are shared by the vast majority ofP. aeruginosa strains, but they are not present in otherPseudomonads.
P. syringae is a prolificplant pathogen. It exists as over 50 differentpathovars, many of which demonstrate a high degree of host-plant specificity. Numerous otherPseudomonas species can act as plant pathogens, notably all of the other members of theP. syringae subgroup, butP. syringae is the most widespread and best-studied.[citation needed]
P. tolaasii can be a major agricultural problem, as it can cause bacterial blotch of cultivatedmushrooms.[38] Similarly,P. agarici can cause drippy gill in cultivated mushrooms.[39]
Since the mid-1980s, certain members of the genusPseudomonas have been applied to cereal seeds or applied directly to soils as a way of preventing the growth or establishment of crop pathogens. This practice is generically referred to asbiocontrol. The biocontrol properties ofP. fluorescens andP. protegens strains (CHA0 or Pf-5 for example) are currently best-understood, although it is not clear exactly how the plant growth-promoting properties ofP. fluorescens are achieved. Theories include: the bacteria might induce systemic resistance in the host plant, so it can better resist attack by a true pathogen; the bacteria might outcompete other (pathogenic) soil microbes, e.g. bysiderophores giving a competitive advantage at scavenging for iron; the bacteria might produce compounds antagonistic to other soil microbes, such asphenazine-type antibiotics orhydrogen cyanide. Experimental evidence supports all of these theories.[40]
Some members of the genus are able to metabolise chemical pollutants in the environment, and as a result, can be used forbioremediation. Notable species demonstrated as suitable for use as bioremediation agents include:
P. putida, which has the ability to degrade organic solvents such astoluene.[50] At least one strain of this bacterium is able to convertmorphine in aqueous solution into the stronger and somewhat expensive to manufacture drughydromorphone (Dilaudid).
One of the most concerning strains ofPseudomonas isPseudomonas aeruginosa, which is responsible for a considerable number of hospital-acquired infections. Numerous hospitals and medical facilities face persistent challenges in dealing withPseudomonas infections. The symptoms of these infections are caused by proteins secreted by the bacteria and may includepneumonia,blood poisoning, andurinary tract infections.[52]Pseudomonas aeruginosa is highly contagious and has displayed resistance to antibiotic treatments, making it difficult to manage effectively. Some strains ofPseudomonas are known to targetwhite blood cells in variousmammal species, posing risks to humans, cattle, sheep, and dogs alike.[53]
WhilePseudomonas aeruginosa seems to be a pathogen that primarily affects humans, another strain known asPseudomonas plecoglossicida poses risks to fish. This strain can cause gastric swelling and haemorrhaging in fish populations.[53]
Various strains ofPseudomonas are recognized as pathogens in the plant kingdom. Notably, thePseudomonas syringae family is linked to diseases affecting a wide range of agricultural plants, with different strains showing adaptations to specific host species. In particular, the virulent strainPseudomonas tolaasii is responsible for causing blight and degradation in edible mushroom species.[53]
One way of identifying and categorizing multiple bacterial organisms in a sample is to use ribotyping.[54] In ribotyping, differing lengths of chromosomal DNA are isolated from samples containing bacterial species, and digested into fragments.[54] Similar types of fragments from differing organisms are visualized and their lengths compared to each other by Southern blotting or by the much faster method ofpolymerase chain reaction (PCR).[54] Fragments can then be matched with sequences found on bacterial species.[54] Ribotyping is shown to be a method to isolate bacteria capable of spoilage.[55] Around 51% ofPseudomonas bacteria found in dairy processing plants areP. fluorescens, with 69% of these isolates possessing proteases, lipases, and lecithinases which contribute to degradation of milk components and subsequent spoilage.[55] OtherPseudomonas species can possess any one of the proteases, lipases, or lecithinases, or none at all.[55] Similar enzymatic activity is performed byPseudomonas of the same ribotype, with each ribotype showing various degrees of milk spoilage and effects on flavour.[55] The number of bacteria affects the intensity of spoilage, with non-enzymaticPseudomonas species contributing to spoilage in high number.[55]
Food spoilage is detrimental to the food industry due to production of volatile compounds from organisms metabolizing the various nutrients found in the food product.[56] Contamination results in health hazards from toxic compound production as well as unpleasant odours and flavours.[56] Electronic nose technology allows fast and continuous measurement of microbial food spoilage by sensing odours produced by these volatile compounds.[56] Electronic nose technology can thus be applied to detect traces ofPseudomonas milk spoilage and isolate the responsiblePseudomonas species.[57] The gas sensor consists of a nose portion made of 14 modifiable polymer sensors that can detect specific milk degradation products produced by microorganisms.[57] Sensor data is produced by changes in electric resistance of the 14 polymers when in contact with its target compound, while four sensor parameters can be adjusted to further specify the response.[57] The responses can then be pre-processed by a neural network which can then differentiate between milk spoilage microorganisms such asP. fluorescens andP. aureofaciens.[57]
Recently,16S rRNA sequence analysis redefined the taxonomy of many bacterial species previously classified as being in the genusPseudomonas.[15] Species removed fromPseudomonas are listed below; clicking on a species will show its new classification. The term 'pseudomonad' does not apply strictly to just the genusPseudomonas, and can be used to also include previous members such as the generaBurkholderia andRalstonia.
^To aid in the flow of the prose in English, genus names can be"trivialised" to form avernacular name to refer to a member of the genus: for the genusPseudomonas it is "pseudomonad" (plural: "pseudomonads"), a variant on the non-nominative cases in theGreek declension ofmonas, monada.[74] For historical reasons, members of several genera that were formerly classified asPseudomonas species can be referred to as pseudomonads, while the term "fluorescent pseudomonad" refers strictly to current members of the genusPseudomonas, as these producepyoverdin, a fluorescentsiderophore.[4][page needed] The latter term, fluorescent pseudomonad, is distinct from the termP. fluorescens group, which is used to distinguish a subset of members of thePseudomonas sensu stricto and not as a whole
^Lalucat, Jorge; Gomila, Margarita; Mulet, Magdalena; Zaruma, Anderson; García-Valdés, Elena (2021). "Past, present and future of the boundaries of thePseudomonas genus: Proposal ofStutzerimonas gen. nov".Syst Appl Microbiol.45 (1) 126289.doi:10.1016/j.syapm.2021.126289.hdl:10261/311157.PMID34920232.S2CID244943909.
^Padda, Kiran Preet; Puri, Akshit; Chanway, Chris P. (2018-09-20). "Isolation and identification of endophytic diazotrophs from lodgepole pine trees growing at unreclaimed gravel mining pits in central interior British Columbia, Canada".Canadian Journal of Forest Research.48 (12):1601–1606.Bibcode:2018CaJFR..48.1601P.doi:10.1139/cjfr-2018-0347.hdl:1807/92505.ISSN0045-5067.S2CID92275030.
^Migula, W. (1894) Über ein neues System der Bakterien. Arb Bakteriol Inst Karlsruhe 1: 235–238.
^Migula, W. (1900) System der Bakterien, Vol. 2. Jena, Germany: Gustav Fischer.
^abAnzai, Y.; Kim, H.; Park, J. Y.; Wakabayashi, H. (2000). "Phylogenetic affiliation of the pseudomonads based on 16S rRNA sequence".International Journal of Systematic and Evolutionary Microbiology.50 (4):1563–89.doi:10.1099/00207713-50-4-1563.PMID10939664.
^Lau, Gee W.; Hassett, Daniel J.; Ran, Huimin; Kong F, Fansheng (2004). "The role of pyocyanin in Pseudomonas aeruginosa infection".Trends in Molecular Medicine.10 (12):599–606.doi:10.1016/j.molmed.2004.10.002.PMID15567330.
^Matthijs, Sandra; Tehrani, Kourosch Abbaspour; Laus, George; Jackson, Robert W.; Cooper, Richard M.; Cornelis, Pierre (2007). "Thioquinolobactin, a Pseudomonas siderophore with antifungal and anti-Pythium activity".Environmental Microbiology.9 (2):425–434.Bibcode:2007EnvMi...9..425M.doi:10.1111/j.1462-2920.2006.01154.x.PMID17222140.
^Hassett, Daniel J.; Cuppoletti, John; Trapnell, Bruce; Lymar, Sergei V.; et al. (2002). "Anaerobic metabolism and quorum sensing byPseudomonas aeruginosa biofilms in chronically infected cystic fibrosis airways: rethinking antibiotic treatment strategies and drug targets".Advanced Drug Delivery Reviews.54 (11):1425–1443.doi:10.1016/S0169-409X(02)00152-7.PMID12458153.
^abRyan, Kenneth J.; Ray, C. George; Sherris, John C., eds. (2004).Sherris Medical Microbiology (4th ed.). McGraw Hill.ISBN0-8385-8529-9.
^Hardie, Kim R.; Pommier, Stephanie; Wilhelm, Susanne (2009). "The Secreted Proteins ofPseudomonas aeruginosa: Their Export Machineries, and How They Contribute to Pathogenesis".Bacterial Secreted Proteins: Secretory Mechanisms and Role in Pathogenesis. Caister Academic Press.ISBN978-1-904455-42-4.
^Brodey, Catherine L.; Rainey, Paul B.; Tester, Mark; Johnstone, Keith (1991). "Bacterial blotch disease of the cultivated mushroom is caused by an ion channel forming lipodepsipeptide toxin".Molecular Plant-Microbe Interactions.1 (4):407–411.doi:10.1094/MPMI-4-407.
^Sepúlveda-Torres, Lycely Del C.; Rajendran, Narayanan; Dybas, Michael J.; Criddle, Craig S. (1999). "Generation and initial characterization ofPseudomonas stutzeri KC mutants with impaired ability to degrade carbon tetrachloride".Archives of Microbiology.171 (6):424–429.Bibcode:1999ArMic.171..424D.doi:10.1007/s002030050729.PMID10369898.S2CID19916486.
^abcdMagan, Naresh; Pavlou, Alex; Chrysanthakis, Ioannis (5 January 2001). "Milk-sense: a volatile sensing system recognises spoilage bacteria and yeasts in milk".Sensors and Actuators B: Chemical.72 (1):28–34.Bibcode:2001SeAcB..72...28M.doi:10.1016/S0925-4005(00)00621-3.
